A Study on the Thermodynamic Mechanism of Asymptotic Frontogenesis Convergence Line and its Strong Rainfall on the Southeast Coast

Abstract:Based on the statistical analysis of heavy precipitation in the west coast of the Taiwan Strait, the winter precipitation data of the European Center in December, January and February 2011-2016 were classified by EOF, REOF and North test, and the REOF mode1 and mode 4 located in the inland were selected. The two-mode precipitation samples were synthesized analyzed respectively，and their precipitation centers were consistent with the rainstorm centers of many years in Fujian. On 1000hpa , there is an asymptotic convergence flow from north to south with a front zone， this forms an asymptotic frontogenesis convergence line with synoptic scale. The strong precipitation is usually located on the left side of the convergence axis where wind with cyclonic shears. This is a type of winter rainstorm system in the southeastern coastal areas that has not been specifically discussed in the past. The structure of the system is confirmed and diagnosed by an objective determination method and establishing a system coordinate system. In the winter when it is relatively cold and dry, and the low-level is prevailing in the northerly wind, such a system combines the thermal lift of the front zone and the dynamic uplift of the convergent airflow to enhance the ascending motion of the rain zone; At the same time, the vapor from adjacent sea surface is collected into the precipitation area by transfer of wind component of convergence line, combined the southerly airflow at the middle and upper levels from subtropic high west edges, they form into a relatively deep vapor transport layer. By non-adiabatic heating, a deep thermal convection unstable is formed. By dry zone wedges under the wet air mass，the wet dynamic instability is formed through the environment of wet upper and dry beneath， and the pseudo-equivalent temperature decreases with height, it forms a convective unstable of cold and dry under warm and wet environment Therefore, the system affects the occurrence and development of heavy precipitation in winter. The influence of environmental thermal mechanism is investigated through the simulation of WRF mode. The results show that the latent heat of condensation can affect the convergence position and strength of the convergence line, the position and strength of the frontal zone, and thus affect the activity of the system. The middle levels is heated by latent heat to inhibit sensible heat cooling into the warm air mass and maintain the thermal instability and precipitation intensity in the rainfall area.The asymptotic frontogenesis convergence line is conducive to the occurrence of heavy rain in the large-scale precipitation during winter in Fujian. In the situation the latent heat release of condensate has an important contribution. The topographical effects of this low-level system will be discussed in detail in another paper.